The present invention pertains in general to electrical circuit simulators and, more particularly, to an electrical circuit simulator for performing noise analysis in the time domain.
During the design of a product, the design engineer takes advantage of all of the design tools available to him in order to evaluate the design prior to actually releasing the design to manufacturing in order to have a prototype manufactured. Prior to the emergence of current electrical simulators, it was necessary to first design an integrated circuit and then evaluate it with the available analysis tools and then release it to manufacturing in order to have the circuit manufactured in the form of a wafer. Typically, this could take two or more iterations, each iteration being a very time consuming process. It was not unexpected in the early days of integrated circuits to take upwards of six months to complete and evaluate a single iteration of a design.
With current day design techniques, the design engineer relies upon simulators to evaluate their circuit design. These simulators have become very sophisticated, such that the design time for a product is significantly decreased and the confidence level in the performance of the circuit is very high after evaluation by the simulator program. It is not uncommon for the xe2x80x9cfirst siliconxe2x80x9d to work substantially as designed. One type of simulator program currently utilized is the SPICE program or a facsimile thereof.
One limitation of current simulators is their ability to evaluate the noise performance of a circuit. For small signal circuits such as amplifiers, the small signal noise analysis can be performed with the simulators. The reason for this is that the simulators perform an AC analysis of the circuit, this analysis always being performed in the frequency domain with the simulated circuit set to fixed bias points on all the components in the circuit, this being the DC operating point of each of the circuits.
The usefulness of the simulation with respect to noise is often limited due to the inaccuracy of the noise model that is utilized. For instance, it is well known that noise models for MOS transistors (thermal and flicker noise) are incomplete and cannot be utilized in some bias regions. For example, if a comparator were being evaluated, it is possible for the DC bias point to vary considerably due to noise considerations. Therefore, traditional noise simulations can only be applied to circuits which operate in small signal conditions and it is therefore not possible to analyze circuits that cannot be simulated in the AC analysis mode for these simulators.
In order to accommodate noise considerations with varying DC operating points, circuits have been analyzed in the time domain utilizing a transient noise analysis. One such approach is disclosed in P. Bolcato and R. Poujois, xe2x80x9cA New Approach for Noise Simulation and Transient Analysis,xe2x80x9d IEEE, 1992, Page 887-890. In the transient analysis disclosed in Bolcato, the noise simulation is performed in time domain of the circuit. In their approach, current sources are introduced into each noisy component, resistors, diodes, MOSFETs, JFETs and BJTs. The sources are designed to represent the noise equivalent current sources of the devices. In simulating the noise sources, an attempt is made to represent the physical noise source of each of the devices with the added noise source. Although it is noted that noise signals could have been generated by sets of random values with controlled variance or amplitude distribution or as continuous signals between two discrete values, Bolcato clearly states that these kinds of signals are known to disturb the normal functioning of the simulator. The approach is to generate a sum of a fixed number of sinusoids by the following equation:       n    ⁡          (      t      )        =            ∑              i        =        1                    N        f              ⁢                            a          i                ⁡                  (          t          )                    ⁢      _sin      ⁢              xe2x80x83            ⁢              (                              ω            it                    +                      ϕ            i                          )            
The reason for utilizing this approach is that it is believed by the author that this will not disrupt the simulator behavior due to the fact that the signals are continuous and fully deterministic. The reason for this is that the simulator operates on discrete samples and is therefore not a continuous simulation. Therefore, the author has opted not to utilize the technique of either generating signals with sets of random values with controlled variance or amplitude distribution or by generating signals that are continuous between two discrete values. By utilizing a fixed number of sinusoids and summing them, there must be some type of iterative technique utilized to generate the sinusoids to result in a theoretical noise spectrum, i.e., this is a deterministic approach. The disadvantage to this type of system is that it does not fully represent the stochastic noise process that exists in a device.
The present invention disclosed and claimed herein comprises a simulator for simulating the noise response of an electrical circuit in the time domain. The simulator operates in discrete time steps TS and includes a matrix of electrical circuit components which make up the electrical circuit. An analyzer is provided for analyzing the transient response of the circuit elements in the time domain with a known input signal to provide a current value for each of the elements. A stochastic noise source is associated with each of the elements, each for generating a stochastic noise current for the associated element. This stochastic noise current represents a stochastic random process comprised of a white noise source scaled by the standard deviation of the physical noise process that exists within the associated element. A summing device is associated with each of the elements for summing the generated noise current for the associated one of the stochastic noise sources with the current values for each of the elements. A matrix solver then solves the matrix after the summing device has performed the associated summing operation.